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of load and generation will result in different transfer limits. Thus, the transfer limit across a given interface can be different every hour (em in fact, will change hour by hour, day to day, season to season, and year to year (em even if there are no changes in the transmission system itself.
Finally, suppose a key transmission line on an interface trips out, and stays out. The transmission transfer capability, which always is determined by the next potential contingency, may now drop significantly: for example, from 3,000 megawatts (Mw) to 1,200 Mw.
Any power in excess of 1,200 Mw flowing over that interface must now be canceled as quickly as possible, since the system is now vulnerable and could be blacked out by a single contingency. (Ten minutes is a common standard today.) If reducing the power flow on the interface means interrupting "firm" transactions, so be it; the integrity of the system demands it. The only alternative is to be extremely conservative in defining capabilities for firm power transactions (em for example, not allowing sale of "firm" above a transfer capability determined by the two worst contingencies.
It takes a lot more effort to determine transfer capabilities then most people realize. Typically, 15 to 20 steady-state load-flow cases, and about a half-dozen transient stability dynamic simulations, must be run to determine a single interface transfer capability for a single load level or season for a single year. If it turns out that an interface has a particular stability problem, perhaps two or three times as many additional cases may have to be run.
Criteria: Region by Region
A look at the reliability criteria for the nine regional reliability councils reveals some significant differences. The different geographies or geo-electric characteristics of each region account for some of these differences, as do variations in population density, which translate into variations in electric demand density. The location of generation resources also affects reliability criteria. Finally, the required level of reliability will vary somewhat (em just compare high-rise New York or Chicago to rural Wyoming or South Carolina!
Different regions will define transfer capability differently, too. The Northeast Power Coordinating Council (NPCC), since it is geo-electrically peninsular to the rest of the Eastern Interconnection, uses interfaces (em groups of parallel transmission lines over which power transfers flow. (In NPCC, most transmission restrictions occur on interfaces internal to the pools or utilities, not on the "seams" between pools or utilities.) In regions more toward the center of the Eastern Interconnection, however, defining system-to-system transfer capabilities may often make more sense than using interfaces. None of these differences should cause concern in themselves, as long as the reliability criteria are applied, and the transfer capabilities defined, in a fair and nondiscriminatory manner.
Solutions: We're Dealing with Physics
Are there effective solutions to these problems? Of course. In fact, there are a number of reasonable paths that can be followed. But it is critically important that all parties unequivocally accept certain key principles. If these principles are not the basis of every rule and procedure, the "brave